The present invention relates to an intake manifold system for a multi-cylinder four stroke spark-ignition internal combustion engine.
The invention seeks to provide an intake system that improves control over the distribution of fuel and air within the charge in the engine combustion chamber.
According to the present invention, there is provided an intake manifold system for a multi-cylinder four stroke spark-ignition internal combustion engine wherein each cylinder has at least one intake port, each intake port being connected at one end to the combustion chamber by at least one intake valve and at the other end to a first intake duct that is connected to a first plenum common to the first intake ducts of other engine cylinders, wherein each intake port is further supplied with gases containing no fuel by a second intake duct having a fixed outlet cross-sectional area equal to between 25% and 45% of the maximum effective through flow cross-sectional area of the open intake valve, the second intake duct being directed towards the closed end of the intake port whereby, during the periods that the intake valve is closed, gas is drawn in through the second intake duct by manifold vacuum and stored in the first intake duct, and wherein the solid boundary walls of the intake port and of the discharge end of the second intake duct define a U-shaped flow path for guiding the gas entering the intake port from the second intake duct, the geometry of the solid boundaries being such that during operation gas enters from one side of the intake port, performs a single U-turn by flowing around the intake valve stem while adhering to the walls of the intake port and enters the first intake duct from the other side of the intake port, the gas being thereby constrained to scavenge the closed end of the intake port and to displace substantially all the gases previously present therein into the first intake duct.
The invention is not the first proposal to blow air into the intake port to improve charge preparation. It is known for example to provide an intake manifold system in which an air assisted fuel injector is positioned near the intake valve to introduce air and fuel into the intake port. Small amounts of air, bypassing the butterfly throttle, have also been introduced into the intake port to assist charge preparation by stirring the air near the intake valve when it is closed. In all such systems, the size of the air jet has to be small in order to give the maximum jet velocity for fine atomising of the fuel or deep penetration for good turbulent mixing. Furthermore, the size of such an air jet cannot be made larger because the flow from it must be limited to less than the engine air intake flow during idling; otherwise it would interfere with the idle speed control of the engine. This limits the size of each air jet to less than 1% of the open intake valve area delivering no more than 80% of the throttled idle air flow to the engine.
In the present invention, the effective flow cross-section of the second intake duct is much larger to give significantly larger flows to satisfy at least 60% of the maximum engine air flow requirement and is not limited by the engine idle operating condition. The flow velocity emitted from the duct is kept low to give an organised flow around the end of the intake port with substantially no turbulent mixing.
It is also known to provide an exhaust gas recirculation (EGR) system in a spark ignition engine. In such an engine, the EGR pipe is conventionally connected to the intake plenum where the exhaust gases can mix thoroughly with the intake air before entering the intake ports. This is because it is not an acceptable practice to introduce the exhaust gases directly into each intake port where there is wet fuel since any fuel entrained within the neat exhaust gases will not find oxygen for combustion and will result in excessive hydrocarbons emissions in the exhaust gases and unstable combustion.
The present invention proposes taking in a major proportion of the intake charge at much higher flow rates under a wide range of operating conditions through the second intake ducts and storing it in the first intake ducts before it is drawn into the cylinders. This has various advantages that stem from being able to control the mixture strength in different parts of the combustion chamber either to achieve homogeneous mixing or to achieve charge stratification.
A problem encountered with fuel injected engines is that the fuel is not well mixed in the charge under all conditions. If fuel is injected into the intake port when the valve is closed, it tends to forms pools and this wall wetting causes hysteresis problems that affect cold starts and the transient response of the engine.
In the present invention, even when the intake valve is closed, the intake port continues to be scavenged by air drawn in through the second intake duct under the action of the manifold vacuum created by the other engine cylinders connected to the same first plenum. This maintains the intake port dry of fuel and creates in the first intake duct a column containing a stratified mixture of fuel and air.
Depending on the geometry of the intake manifold, in particular the length of the first intake duct, this column can either be stored ready to be introduced back into the same cylinder when the intake valve next opens or the mixture may enter the first plenum to mix thoroughly with the mixture drawn from other cylinders. If the charged is stored as a stratified column in a long duct, then when the intake valve opens, the column is transferred into the cylinder and determines the charge stratification within the cylinder. On the other hand if the first ducts are short, not only is better charge preparation achieved in each cylinder but homogeneity between cylinders is improved.
The ability to store a stratified column allows the invention to accentuate the inhomogeneity in the cylinder and thereby permit the engine load to be regulated at least in part by modifying the amount of fuel introduced into the cylinder (rather than the amount of fuel and air mixture) while introducing an excess of air into the engine. This permits losses associated with pumping air in the intake manifold to be reduced.
In an extreme case, it is possible to concentrate all the fuel at one end of cylinder as a homogeneous easily ignitable mixture while filling the lower end of the cylinder with air or exhaust gases into which the flame cannot propagate. Such a divided charge may be used to achieve very lean overall fuel-air mixture or very high overall EGR dilution. If air forms the lower part of the divided charge and a very rich fuel-air mixture forms the upper part, then such division can create incomplete combustion in the upper part of the combustion chamber while still delivering an overall stoichiometric mixture to a catalytic converter or an afterburner. The completion of combustion of such an exhaust gas mixture in the exhaust system can assist in raising the catalytic converter to its light off temperature. If exhaust gases form the lower part of the divided charge and a stoichiometric fuel and air mixture forms the upper part, then the engine can be run with high overall dilution thereby reducing the volumetric efficiency without disturbing the stoichiometry of the exhaust gases.
It should be mentioned that there have been disclosed systems in the prior art in which two intake ducts lead to each intake port and these have generally had one of three aims in mind.
The first aim of the prior art proposals, as exemplified by EP-A-0 098 543, is to achieve manifold tuning by matching the length of the intake ducts to the engine speed, a longer and narrower duct being used at low engine speeds and a shorter and wider duct at high engine speeds. The effect of flow reversal in the first ducts when the intake valve of a cylinder is closed is not appreciated, nor is the design of the intake port optimised by using the reverse flow to scavenge the closed end of the intake port and achieve charge mixture stratification control.
The second aim, as exemplified by GB-A-1,195,060 and U.S. Pat. No. 4,867,109, is to achieve flow balancing. In these systems, in addition to the conventional intake ducts leading from the intake throttle or throttles, there are interconnecting ducts that balance the flow between the cylinders. The flow through these balancing ducts will reverse directions as occurs in the first intake ducts of the present invention but while the first intake ducts in the present invention occupy the major cross sectional area of the intake ports, the balancing ducts in the prior art have a much smaller cross section. Because of the size and location of these balancing ducts the gases flowing into them play little part in scouring the closed end of the intake port or in transporting any significant volume of gas out of the intake port.
The last aim of the prior art in providing two ducts to each intake port, as exemplified by GB-A-2,114,221, GB-A2,038,415 and GB-A-1,239,264, is to achieve high intake air velocity to increase turbulence at low engine speeds by using a smaller supply duct at low engine speeds and a larger supply duct at high speed. These proposals differ from the present invention in that, in all three cases, the smaller duct is required to supply premixed fuel and air, it being optional to provide fuel in the larger duct. In the present invention, the scouring of the intake port must be carried out by gases containing no fuel as the purpose of the scouring is to achieve charge mixture stratification.
EP-A-0 076 632 falls partially into both of the above latter two categories. Here a variable air jet supplying air from the intake duct across to a balancing port is used to induce a continuous vortex within the intake port while the intake valve is closed. However detailed study has shown that this cannot achieve complete scavenging of the intake port as required in the present invention. Without a solid boundary extending substantially to the axis of the cylindrical space above the intake valve head to guide the U-flow to scour or scavenge the closed end of the intake port, it is found that the flow from the air jet follows one of two possible stable flow patterns depending on the width of the jet. The first flow pattern is created with a narrow high velocity jet entering from one side of the intake port and inducing within the volume of the intake port a strong vortex, this being the aim of this prior art invention, but most of the content in the intake port merely rotates and remains inside the port. The second flow pattern is created with a wider lower velocity jet which initially tends to follow a U-flow path around the closed end of the intake port, but because of the absence of a solid boundary separating the forward and reverse flows, shearing action soon causes the deep U-flow pattern to collapse as the gas mixes into itself and the final flow pattern becomes one in which the air turns around in a shallower U-flow path near the immediate vicinity of the discharge boundary of the air jet, leaving the volume further inside the intake port undisturbed or circulating in counter rotation with the shallower U-flow. Here again the content at the end of the intake port is not scavenged.
It is found that there is a narrow range of the width of the air jet which can sustain the required deep U-flow pattern at the end of the intake port for a short period of time, even without the solid boundary specified in the present invention. However this is unstable and the flow pattern reverts to one or the other stable flow patterns during the engine cycle and in an unpredictable manner from one cycle to the next. This geometry is therefore not practical for any serious application because it fails to provide reliable and complete scavenging of the intake port which is the necessary condition in order to satisfy the main aim of the present invention for achieving the desired charge mixture stratification.
The consequence of not completely scavenging the content of the intake port during every engine cycle is that clean combustion cannot be guaranteed with the charge mixture stratification of this invention. In the case where liquid fuel is introduced as a spray into the intake port by means of a fuel injector, some of the liquid will be deposited on the back of the intake valve. Whereas a strong scavenging flow scouring the end walls of the intake port will be very effective in evaporating the fuel and transporting the vapour out of the port, a stagnant or recirculating flow would leave a pocket of liquid fuel or saturated vapour remaining inside the port. In the case where a premixed fuel-air mixture is supplied from the first intake duct and exhaust gas is introduced through the second intake duct as the scavenging gas for clearing the port of any fuel-air mixture left behind from the previous intake stroke, a pocket of fuel-air mixture would once again be retained in the port if the scavenging flow is poorly directed. In both these case, the unscavenged pocket of fuel will be drawn into the combustion chamber first at the beginning of the next intake stroke. Because of the organised bulk motion intentionally created in the combustion chamber to retain the charge stratification of the column of mixture drawn in during the course of the intake stroke, this first pocket will reside in a region of the combustion chamber which is disconnected from the main mixture charge by a layer of air or exhaust gas. Such pocket will not be burned with the main charge and will be discharged as increased exhaust emissions and cause increased fuel consumption.
In summary, none of the prior art has appreciated the possibility of controlling the charge mixture stratification inside the engine combustion chamber by controlling the charge mixture stratification inside the intake port and manifold system which gives clean combustion by positively guiding the U-flow to scour the closed end of the intake port thus ensuring stable and complete scavenging of the port during every engine cycle. Previous systems have overlooked the important details of the geometry of the intake port and of the discharge boundary of the second intake duct which are crucial in determining the success of the present invention, but are not recognised because the aims of those systems are concerned with parameters other than charge mixture stratification.